1. Field of the Invention
The present invention relates to a method of manufacturing a grain-oriented silicon steel sheet exhibiting excellent magnetic characteristics, and, more particularly, a method of stabilizing the magnetic characteristics in the lengthwise direction of a coil of a grain-oriented silicon steel sheet.
2. Description of the Related Art
Grain-oriented silicon steel sheet is used in transformer cores, generators and the like, and therefore requires excellent, magnetic characteristics such as high magnetic flux density (usually indicated by value B.sub.8 at a magnetic-field intensity of 800 A/m) and small iron loss (usually indicated by 50 Hz alternating iron loss value W.sub.17/50 at the maximum magnetic flux density of 1.7 T).
Much work has gone into minimizing iron loss in grain-oriented silicon steel, and improvements have resulted from (1) reducing the thickness of the steel sheet, (2) increasing Si content, and (3) reducing the diameters of crystal grains. Such steps have enabled the production of a material that exhibits an iron loss W.sub.17/50 of only 0.90 W/kg.
However, reducing iron loss even further has proven difficult because further reductions in the steel sheet thickness causes defects to arise during secondary recrystallization, thus increasing iron loss. Similarly, reducing crystal grain diameters below an average diameter of about 4 mm to 8 mm also causes iron-loss-increasing defects to arise during secondary recrystallization. Moreover, increasing Si content negatively affects the ease with which cold rolling can be performed.
However, by using a so-called magnetic domain refining technique in which a local distortion is introduced into the surface of the steel sheet, or grooves are formed on the same, iron loss can be considerably reduced.
That is, in the case of the foregoing material having an iron loss W.sub.17/50 of 0.90 W/kg, introduction of appropriate local distortion on the surface of the steel sheet (by a plasma jetting method or the like) has reduced iron loss of 0.80 W/kg. This magnetic domain refining technique also eliminates the need to reduce crystal grain diameters in the final product, as is required in conventional techniques. The quality of material produced through the magnetic domain refining technique depends upon the thickness of the steel sheet, the Si content, and the magnetic flux density.
Since Si content cannot be increased without negatively affecting the working properties necessary for the steel, minimization of iron loss requires increasing the magnetic flux density of a thin material.
To improve the magnetic flux density of a grain-oriented silicon steel sheet, the orientation of crystal grains of the product must be highly integrated in orientation (110) [001], known as the Goss orientation. Such Goss oriented grains can be obtained through a secondary recrystallization phenomenon created during a final annealing process.
In such a secondary recrystallization, selective crystal grain growth is promoted in crystal grains having the orientation (110) [001], while growth of crystal grains in other orientations is minimized by adding an inhibitor. The inhibitor forms a fine deposited and dispersed phase in the steel, thereby selectively inhibiting growth of grains.
Since the selective growth of Goss oriented grains produces a material exhibiting high magnetic flux density, there has been much research and development regarding inhibitors. A particularly effectively AlN inhibitor has been disclosed in Japanese Patent Publication No. 46-23820, wherein a steel sheet containing Al is subjected to a rapid cooling process after it has been annealed but before a final cold rolling process is performed. The final cold rolling is performed using a high rolling reduction ratio of 80% to 95% to produce a steel sheet having a thickness of 0.35 mm and a high magnetic flux density B.sub.10 of 1.981 T (B.sub.8 of about 1.95 T).
However, steel sheet produced by the above-described method suffers from the problem that high magnetic flux density cannot be maintained when the sheet thickness is reduced.
That is, (110) [001]oriented grains, which form the nuclei of the secondary recrystallization, are not distributed uniformly in the direction of the thickness of the steel sheet. Instead, the grains are concentrated near the surface layer of the steel sheet. Therefore, if the thickness of the sheet is reduced, (110) [001]orientated grains are readily affected by the atmosphere in which the final annealing process is performed, such that the secondary recrystallization becomes unstable. Thus, a method of stabilizing the magnetic characteristics has been widely sought after.
Accordingly, a variety of techniques for manufacturing grain-oriented silicon steel sheet having excellent and table magnetic characteristics have been developed. For example, a technique in which an aging heat treatment is performed at 50.degree. C. to 350.degree. C. for one or more minutes during the rolling process (Japanese Patent Publication No. 54-13846), a technique in which the steel sheet is maintained at 300.degree. C. to 600.degree. C. for 1 to 30 seconds during the cold rolling process (Japanese Patent Publication No. 54-29182) and a warm rolling technique in which the temperature of the inlet portion of the rolling stand is controlled to 150.degree. C. to 300.degree. C. have all been developed. However, all of the foregoing techniques are unsatisfactory methods for manufacturing industrial products because, while the coils manufactured from steels made in accordance with the above-described techniques exhibit excellent magnetic characteristics at either end of the coils (the leading and tailing ends of the steel), the magnetic characteristics in the central portion of the coil are deteriorated.
As described above, if a warm rolling process (for raising the temperature of the steel sheet) or an aging heat treatment is performed during the cold rolling of a grain-oriented silicon steel sheet containing Al, the magnetic flux density markedly deteriorates except the two ends of the product.
After investigating the foregoing problem, we discovered that, although the secondary recrystallization is completed in all regions of the product, the orientation of the crystal grains in the regions in which the magnetic flux density deteriorates departs considerably from the orientation (110) [001].
As shown in FIG. 1, the measured change in the angle of deviation in plane from the orientation [001](the "angle of deviation" is hereinafter referred to as "angle .alpha.") increases except the two ends of the coil, thus causing the magnetic flux density to be lowered.
This phenomenon occurs when cold rolling is performed at a warm temperature range from about 100.degree. C. to 300.degree. C., or when an aging or heat treatment is performed during the rolling process. The foregoing phenomenon often takes place in inverse proportion to the thickness of the steel sheet.